SUBSTATION CABLE VOLTAGE RATING BASIC INFORMATION AND TUTORIALS

The selection of the cable voltage rating
is based on the service conditions of 2.1, the electrical circuit frequency,
phasing, and grounding configuration, and the steady-state and transient
conductor voltages with respect to ground and other energized conductors.

A voltage rating has been assigned to each
standard configuration of shield and insulation material and thickness in NEMA
WC 3-1980, NEMA WC 5-1973, NEMA WC 7-1988, NEMA WC 8-1988, and in AEIC
CS5-1987, AEIC CS6-1987, and AEIC CS7-1987.

The selected voltage rating must result in
a cable insulation system that maintains the energized conductor voltage,
without installation breakdown under normal operating conditions.

For high-voltage cables, it is usual
practice to select an insulation system that has a voltage rating equal to or
greater than the expected continuous phase-to-phase conductor voltage. The NEMA
standards provide for a cable voltage rating that is only 95% of the actual
continuous voltage.

For solidity grounded systems, it is usual
to select the 100 Percent Insulation Level, but the 133 Percent Insulation
Level is often selected where additional insulation thickness is desired. The
133 Percent Insulation Level is also applied on systems without automatic
ground fault protection.

Distribution substations often utilize
cable for the distribution circuits from the substation secondary switch-yard
(substation getaways). The insulation system selected for this distribution
cable may have a voltage rating that is a class above the minimum NEMA rating
for the actual circuit voltage and ground fault protection, because it is
believed that the additional insulation will result in a lower probability of
insulation failure.

Research conducted by the Electric Power
Research Institute has led to cable construction recommendations published in
EPRI EL-6271 [B10].11 The EPRI recommendations for cable insulation systems
have insulation thickness that are the same as those of the NEMA and AEIC
standards.

For power and control cables applied at 600
V and below, some engineers use 1000 V-rated insulation because of past
insulation failures caused by inductive voltage spikes from de-energizing
electromechanical devices, e.g., relays, spring winding motors.

The improved dielectric strength of today's
insulation materials prompted some utilities to return to using 600 V rated
insulation for this application. Low voltage power and control cable rated 600
V and 1000 V is currently in use.

The selection of the power cable insulation
system also includes consideration of cost and performance under normal and
abnormal conditions. Dielectric losses, resistance to flame propagation, and
gas generation when burned are the most common performance considerations.